Gas separation by adsorbent membranes is a new separation method which exhibits superior performance characteristics for many gas mixtures compared with well-known separation methods using polymeric or porous bulk diffusion membranes. Adsorbent membranes are particularly useful for the recovery of light gases such as hydrogen from mixtures containing other components, wherein these other components selectively permeate through the membrane and hydrogen is recovered as a non-permeate product at a pressure only slightly below the feed pressure. By comparison, polymeric membranes operate in an opposite mode in which the hydrogen selectively permeates through the membrane and is recovered at a pressure far lower than the feed pressure, and other components are recovered in a non-permeate stream at a pressure slightly below the feed pressure. The characteristics and methods of making adsorbent membranes are described in U.S. Pat. No. 5,104,425 which is incorporated herein by reference.
Staged membranes in series are disclosed in the art in which either the permeate or non-permeate stream from a first stage is passed to one or more additional stages to improve efficiency and/or recovery of the component being recovered. U.S. Pat. No. 4,180,388 discloses a two-stage membrane system in which the non-permeate stream from the first stage passes to a second stage for additional permeate recovery. The feed to permeate pressure ratio in the first stage is lower than the feed to permeate pressure ratio of the second stage. Polymeric membranes are taught for use in the process. U.S. Pat. No. 4,435,191 teaches the use of three membrane stages in series for recovering an aggressive gas such as CO.sub.2 by permeation through polymeric membranes. The feed through the stages is compressed between the stages such that each stage operates at a successively higher pressure on the feed side. U.S. Pat. No. 4,623,704 discloses the recovery of an enriched ethylene stream from polyethylene polymerization off-gas containing unreacted ethylene. The recovered unreacted ethylene stream comprises the combined permeate streams from three membrane modules in series, each module having a solid semipermeable membrane such as cellulose acetate. U.S. Pat. No. 4,104,037 teaches a multi-stage gaseous diffusion system for separating uranium isotopes in the form of gaseous fluorides. The diffusion stages utilize porous membranes which separate gases by molecular weight using the mechanism of bulk pore diffusion. U.S. Pat. No. 3,713,271 discloses the recovery of helium from natural gas using multi-stage membrane permeators in which helium selectively diffuses through the membrane material. Other components in the gas, namely nitrogen and methane, have very low permeability through the membranes and are withdrawn as non-permeate byproduct. U.S. Pat. No. 2,540,151 teaches the separation of oxygen from air in the multiple-stage membrane system using polymeric membranes in which the permeate from a given stage is compressed and passed to the feed side of the succeeding stage. Non-permeate product from a given stage is recycled as feed to the preceding stage. Final products are an oxygen-rich stream and an oxygen-lean stream.
Each background patent discussed above is characterized by at least one of the following features: (1) each stage is operated at a different pressure ratio (defined as the ratio of feed pressure to permeate pressure); (2) only one permeate product and one non-permeate product are withdrawn, and staging is used only to increase the recovery or purity of either the permeate or non-permeate stream; and (3) the membrane material used is either a polymeric membrane or a porous membrane which separates gases based on molecular weight by bulk pore diffusion.
In the background art described above, the term "stage" is used to define two or more membrane zones arranged in series, but the term is applied in several different contexts with respect to the relationship among the stages. U.S. Pat. No. 4,180,388 cited above describes a two-stage system in which non-permeate gas from a first stage passes as feed to a second stage, and a separate permeate stream is withdrawn from each stage at a pressure different from the permeate from other stages. A similar three-stage system is described in U.S. Pat. No. 4,435,191 in which three separate permeate streams are withdrawn from the three stages. U.S. Pat. Nos. 2,540,151 and 4,104,037 cited above both utilize multistage membranes in series wherein the feed gas to a given stage consists of non-permeate gas from the preceding stage combined with permeate gas from the succeeding stage. Thus in the background art described herein the term "stage" is used in several different contexts with respect to the process relationships among the stages.
The use of a membrane permeator with two different membrane materials is described in a paper entitled "Ternary Gas Mixture Separation in Two-Membrane Permeators" by A. Sengupta and K. K. Sirkar in the AIChE Journal, Vol. 33, No. 4, April 1987, pp. 529-539. The use of two different membrane materials allows the separation of ternary gas mixtures into three different products.
The separation of gas mixtures by combinations of membrane systems and pressure swing adsorption (PSA) systems are disclosed by representative U.S. Pat. Nos. 4,398,926; 4,654,063; 4,690,695; 4,701,187; 4,717,407 and 4,836,833. Each of these patents is characterized by at least one of the following features: (1) only single stage membrane systems are utilized; (2) the membrane system feed is obtained from a PSA system; and (3) light components, i.e. helium or hydrogen, preferentially diffuse through the membrane which indicates the use of a polymeric membrane.
Improved methods of separating gas mixtures by membranes and membrane/adsorption combinations will enable the separation of important industrial gas mixtures to be realized more efficiently and economically. Such advances in the gas separation art will benefit diverse applications including oxygen and nitrogen production, helium recovery, hydrogen recovery, hydrocarbon fractionation, synthesis gas production, and other economically important gas-related processes. The improved separation methods disclosed in the following specification and defined by the appended claims advance the gas separation art and offer more efficient means for separating industrially-important gas mixtures.